Hey Matt
Yes I have changed the ignition switch with a new part (lower part that does the switching) and I have swapped all Ignition relays with ones adjacent to each of them with no result. I did notice in checking these that I do not have the Auxiliary Ignition relay in the trunk - don't know why? But the car has run for the last 8 years I've had without one??

Yesterday I thought the problem had cured itself. I had left the car for a couple of hours with the negative lead connected to the positive terminal to drain out any residual power from the ECU and anywhere else it might be lurking.
When I connected the battery (with ignition off) and went to start the car - the car behaved normally - normal ignition on lights, system check executed all good, car started - all lights off and gauges working - not even a check engine light. Was able to start it 6 times without any issues. Left the car for a couple of hours - again it started 6 or 8 times completely normally. In the past - leaving it for anything more than 10 minutes would have resulted in the problem.
I have checked and cleaned all the major earthing points.
In response to Matt's specific question - EVERY time you turn the ignition on BEFORE connecting the battery - it always works fine. Turn it off and back on again (with the battery connected) and you're back to square one. Disconnect the battery and reconnect it - all fine.
I can hear your collective brains whirring out there - and I am confident that sooner or later someone is going to suggest something that solves the problem.
For my part - I think I'm going to take it to the nearest Dealer (two-and half hours away!) and try and replicate the problem while it's hooked up to the computer system.
As the x-files say - 'the answer is out there....'
Thanks to all for your continued interest and contributions.
Gareth

 

Wow, I sure hope you can duplicate the problem at the dealer, and let us know what the solution to the mystery is !

 

How long were you letting it run between your starts /stops /restarts? That many starts in quick succession could draw your battery back down to the point of reintroduced the problem.

 

Well that's a good point I guess but wouldn't explain having the same issue after one start following a full charge. However, I will put the battery on charge tomorrow and try after a few hours. Battery is a brand new AGM, so should recover quickly and certainly would charge fully over a few hours.
Thanks for the suggestion.

 

Okay - so charged battery for 16 hours reading 12.85 volts when charger taken off. Connected the battery - Voltage at 12.80 volts.
Turned ignition on all normal - started engine all normal no warnings of any kind.
Turned off after 20 mins - tachometer, fuel gauge, temp gauge, oil pressure gauge and voltmeter all stay on/same level they were when engine was running.
Turned ignition back on - no lights - and gauges stay the same - start the engine - no change - turn off ignition - no change.
Disconnected battery - reading 12.8v - all gauges stay same - connected negative cable to positive lead - all gauges stay same. Leave for 5 minutes - same!!
Connect negative lead and and all gauges drop back to zero!?
This behavior is screaming Relay Problem to me - but I have swapped all the ignition relays I can find with ones of similar type from another circuit with no result.
This is nuts!!

 

Final post for this thread - a very unsatisfactory end, in that everything has been fine for the last 6 days running the car for a few miles (around 15) each day, and the problem has gone away!!
I want to know why? Certainly the battery seems to maybe have been at fault, but nothing definitive since it also failed when the battery was fully charged giving 12.8v at rest.
Perhaps the EMS just needed a few dozen miles or so to re-stabilize - but again - nothing definitive.
Well anyway, it's time to put the old girl on the market, as I am moving back to the UK and need a model with the steering wheel on the "Right" side.
Thanks as always, to everyone who participated in trying to solve this tricky puzzle.
We can only wait and see if the problem returns.